I came out to this topic searching for some information on the dielectrical constant of salt water since that approaches the human body. To make a device detecting the approximity of human body mass.. instead of the air.. Presumeably it should be working nice, though I believe the dielectrical constant changes over the frequency domain. So I should be looking for the right electromagnetic frequency to have the biggest dielectrical constant having the biggest response, since air(relative dielectric 1) should have a rather flat dielectrical constant over the frequency domain.

Dielectric constant and resistivity are NOT the same things. One is imaginary, one is real. Surprisingly, sea water and fresh water have ABOUT the same dielectric constant...but very different resistivities, of course.

The Navy did extensive testing on this, for ELF radio experiments. I'll try to dredge up the studies for your. It was actually pretty interesting,

Presumeably it should be working nice, though I believe the dielectrical constant changes over the frequency domain. So I should be looking for the right electromagnetic frequency to have the biggest dielectrical constant having the biggest response

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Yes it does change over the frequency domain; however note that the permittivity of the human body is complex with a relative dielectric constant and a dielectric loss factor, which in a more practical sense manifests itself as the conductivity, and will be discernible for water particularly with high salinity. Note: both these parameters are independent functions of frequency (and temperature too).

Dielectric constant and resistivity are NOT the same things. One is imaginary, one is real. Surprisingly, sea water and fresh water have ABOUT the same dielectric constant...but very different resistivities, of course.

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KL7AJ has referred to it as resistivity, where as I have referred to it as conductivity - they account for the same physical aspect where: